Motor vehicles are today increasingly equipped with radar systems, in order to detect the distance of objects from the motor vehicle and the relative speed of said objects in reference to the motor vehicle. The detected distances and relative speeds may be used in various drive-assistance systems. Such drive-assistance systems may represent for ex-ample automatic light-width controls, automatic selections of light distribution, an automatic adjustment of the vertical or horizontal light-dark boundary, brake assistance, or the like. In such radar systems the device mentioned at the outset may be used comprising a voltage-controlled oscillator.
The European patent publication with the publication number EP 1 325 350 B1 discloses a method and a device for determining the distance and relative speed of an object distanced from the motor vehicle. The patent publication suggests a FMFSK-method (frequency modulated frequency shift keying). The device used in this method comprises a voltage-controlled oscillator, by which several signals to be transmitted are generated. Each signal has several sections with different frequencies, which are constant during the (individual) section, though. The sections are transmitted consecutively, with the frequency of the signal sections of a signal being increased from one section to the next. The sections of a signal are not transmitted directly after each other, though. Rather, each section of one signal is followed by a section of another signal. The alternation between the signal sections occurs here always in the same sequence and is repeated in each clock. This way, boxed signal sections are yielded. The frequency is always kept constant during a signal section.
From the disclosure with the publishing number DE 10 2009 048 112 A1 an alternative method is known, in which the frequency of a signal is not kept constant during a signal section, contrary to the method known from the patent publication EP 1 325 350 B1, but increases or drops linearly. A respective method is also known from the patent publication with the publishing number EP 0 312 601 B1.
In order to yield a result as good as possible in this method, only voltage-controlled oscillations can be used showing certain basic features, which particularly relate to incline, curvature, aging, and temperature drift of the voltage-controlled oscillator. When producing a device as described in these documents, here all voltage-controlled oscillators are measured prior to assembly and only suitable oscillators with the same features are selected.
Additionally, during operation regularly cycles are inserted into the ongoing operation in which the voltage-controlled oscillators are calibrated in order to compensate a temperature drift, for example. The calibration occurs via a phase control loop (also called phase-locked loop (PLL)). The phase-locked loop provided for calibration comprises a circuit arrangement to control the oscillator, namely to adjust the voltage in order to control the oscillator.
During the calibration cycles the devices cannot be used for generating radar signals to measure the distance and relative speeds. This is disadvantageous for the drive-assist systems in that the drive-assist systems cannot be provided with real-time data from the road during said period. This problem has already been discussed in the German patent application of Hella KGaA Hueck & Co. with the reference character DE 10 2010 061 041 dated Dec. 6, 2010.
The application DE 10 2010 061 041 discloses that the circuit arrangement comprises a digital signal processor for controlling the oscillator, which determines a value and provides it digitally at an output and that the digitally provided value at the outlet of the signal processor is converted via a digital-analog converter into an analog voltage signal, which is fed to a voltage-controlled oscillator. It is not described in the application how the signal is structured at the input or output, particularly what progression the signal shows at the input or output of the oscillator or how the input signals are generated.